The present invention relates to nanometerology and in particular to calibration methods, calibration standards, and systems for critical dimension scanning electron microscopy.
In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these higher device densities there have been, and continue to be, efforts toward scaling down the device dimensions on semiconductor wafers. In order to accomplish higher device densities, smaller and smaller features sizes are required. These may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, and surface geometry of corners and edges of various features.
High resolution lithographic processes are used to achieve small features. In general, lithography refers to processes for pattern transfer between various media. In lithography for integrated circuit fabrication, a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the resist. The film is selectively exposed with radiation (such as optical light, x-rays, or an electron beam) through an intervening master template, the mask, forming a particular pattern. Exposed areas of the coating become either more or less soluble than the unexposed areas (depending on the type of coating) in a particular solvent developer. The more soluble areas are removed with the developer in a developing step. The less soluble areas remain on the silicon wafer forming a patterned coating. The pattern corresponds to the image of the mask or its negative. The patterned resist is used in further processing of the silicon wafer.
At various stages in forming the patterned resist coating and processing the silicon wafer, it is desirable to measure critical dimensions resulting from the lithographic process. Critical dimensions include the size of features in the wafer or patterned resist such as line widths, line spacing, and contact dimensions. Due to the extremely fine patterns involved, scanning electron microscopy (SEM) is often employed to analyze critical dimensions. Specialized critical dimension measuring SEM systems have been developed for use with silicon wafers, which are of a size that makes them too large for most SEM systems.
In SEM, an electron beam is scanned across the sample. The beam interacts with the sample to produce measurable responses that vary with position over the course of a scan. Measurable responses include backscattering of electrons and production of secondary electrons, auger electrons, X-rays and cathodoluminescence. Secondary electrons are the most useful of the measurable responses in accessing surface topography and are the responses most often employed in critical dimension analysis.
Although SEM systems measure critical dimensions with high precision, they must be calibrated frequently for the measurements to be accurate. Precision refers to the capability of distinguishing small differences in dimension. Accuracy refers to the correctness of measurements in absolute terms. Precise measurements are reproducible, but contain systematic errors that must be quantified and taken into account for the measurements to be accurate. Calibration quantifies systematic errors and is carried out on a regular basis in SEM systems, usually at least once a day.
Calibration involves taking measurements on a calibration standard. A calibration standard is a sample having accurately known dimensions. One calibration standard commonly employed is a periodic pattern formed into a silicon substrate. Such a calibration sample is simple, but has low contrast and easily becomes contaminated over the course of extended use.
Another type of calibration standard is formed with a patterned polysilicon coating over a silicon wafer. A thin layer of silicon oxide is used to facilitate binding between the patterned polysilicon and the wafer. A similar calibration standard is formed with a uniform polysilicon coating over the silicon oxide layer and has a calibration patterned formed in another silicon oxide coating that is formed over the polysilicon. These calibration standards can be used with very low electron beam energies, however, due to the insulating properties of the silicon oxide, at higher beam energies these calibration standards undesirably accumulate charges that affect the electron beam and skew calibration measurements.
Another calibration standard, described in Yang et al., U.S. Pat. No. 6,048,743, includes a semiconductor wafer, an insulating first patterned layer formed on the wafer, a plurality of contacts electrically communicating with the wafer and formed between the pattern of the first insulating layer, a conductive layer formed over the first insulating layer and in electrical communication with the wafer through the contacts, and a second insulating layer with a second pattern formed over the conductive layer. The conductive layer electrically communicates between the second insulating layer and the wafer and permits charges to drain from the second insulating layer to the wafer during scanning. An example is provided in which the second insulating layer is polysilicon.
During calibration scanning, the electron beam causes carbon to deposit on the calibration standard. Deposited carbon changes the sample dimensions, thus affecting the accuracy of the calibration. While carbon deposition can be dealt with by periodic replacement of the calibration standard, this raises the cost of the calibration standard. There remains an unsatisfied need for SEM system calibration standards and calibration methods with high accuracy an low cost.
The present invention provides SEM calibration standards, and associated SEM systems and SEM calibration methods, that are self-cleaning with respect to electron beam deposited monolayer of carbon contaminents. The calibration standards have coatings containing a transition metal oxide. The coatings facilitate oxidation of deposited carbon, whereby carbon buildup can be stopped or reversed. By providing a mechanism to mitigate carbon buildup, calibration standards provided by the present invention achieve high accuracy, high durability, and low cost.
One aspect of the invention provides a calibration standard for a SEM system including a substrate having a surface on which are formed patterned features having dimensions suitable for calibrating the SEM system, wherein the patterned features include a coating of a material containing a transition metal oxide.
Another aspect of the invention provides a calibration standard for a SEM system including means for removing carbon deposits that form on the calibration standard during calibration scanning.
A further aspect of the invention provides a method of calibrating a SEM system including obtaining a calibration measurement by employing the SEM system to measure a dimension of a feature of a calibration standard, wherein the feature includes a coating of a material containing a transition metal oxide and using the calibration measurement to calibrate the SEM system.
A further aspect of the invention provides a SEM system including a scanning electron microscope and a calibration standard having patterned features including a coating containing a transition metal oxide, wherein the SEM system is configured to employ the calibration standard in calibrating the scanning electron microscope.